(1) Field of the Invention
The present invention relate to the generation of audible tones and particularly to the electronic synthesis of polyphonic music. More specifically, this invention is directed to tone generators for producing an output signal which, when applied as the control input to a sweep voltage generation circuit, will result in a signal in the audible range which varies in a polyphonic manner. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
A tone generator for use in the generation of audio frequency signals in response to the operation of a pedal on an electronic organ is disclosed in U.S. Pat. No. 4,127,048. In the tone generator of the referenced patent a binary number is allocated to each pedal key. These binary numbers are stored in a memory which will function as a down counter. Upon operation of a pedal, the memory is counted downwardly by the output of a clock pulse generator and the carry pulses are used as the output of the tone generator. Thus, the binary number associated with each pedal key serves as a divisor. Submultiples of the output frequency may be derived from the carry pulses by division. While U.S. Pat. No. 4,127,048 does not so state, it is believed that the design and operation of the down counter which functions as the memory therein is in accordance with the teachings of the publication entitled "Frequency synthesizers- 2" which begins at page 75 of WIRELESS WORLD, October 1978.
The state-of-the-art as represented by U.S. Pat. No. 4,127,048 assumes that the player will actuate only a single key. Thus, in actual practice, preference or auctioneering circuits will typically be provided to prevent interferences should a plurality of keys be simultaneously actuated. Circuits such as that described in U.S. Pat. No. 4,127,048 thus do not permit the generation of polyphonic sound.
It is to be noted that digital tone generators are known in the art which permit polyphony. An example of such a digital tone generator may be seen from U.S. Pat. No. 4,217,802. The circuit of U.S. Pat. No. 4,217,802 has the capability of virtually unlimited polyphony because an individual data storage means is provided for each recognizable frequency of the chromatic scale. These plural data storage means are read sequentially in time multiplex. Thus, the generated audio signal may contain information relating to all tones which may be played on the instrument. A major drawback of a digital tone generator of the type of U.S. Pat. No. 4,217,802 is that it is comparatively complex and thus relatively expensive.